Ion beam etching (IBE) and reactive ion beam etching (RIBE) techniques have been used in research and multiple niche applications over the last 15-20 years. In the last five years, “ion milling” techniques have been extensively employed in the manufacture of lead overlay structures, such as thin film magnetic heads (TFMHs) for the data storage industry. The important advantages of ion milling over other etching techniques are excellent etch uniformity and control of etch feature profiles.
Recent trends in the TFMH industry, including shrinkage of the head (slider) form factor and the introduction of high-sensitivity magnetoresistive read elements, have driven the need for anisotropic etching techniques with high selectivity and/or etch control combined with high throughput, reproducibility (repeatability), and yield. Until recently, high-throughput production operations and the deployment of RIBE processes have been hampered by the short filament life and reliability problems of the standard Kaufmnan-type ion sources, particularly when operated with high concentrations of reactive gases. These problems have been eliminated with the introduction of filamentless ion sources, such as the RF inductively coupled plasma (RF-ICP) ion source.
The most competitive alternative technique is reactive ion etching (RIE), which is widely used in the semiconductor industry. In RIE, the substrate is mounted inside the plasma reactor and directional etching is accomplished by applying an electrical bias to the substrate. This allows, in principle, high etch rates and selectivity using reactive plasmas. Optimum results are obtained for RIE processes when the surface to be etched is reacted with a chemical species in the plasma to form volatile reaction products which are pumped out of the system. However, data storage device materials are not highly reactive, and the reactive gas chemistries that have been found for these materials do not easily produce volatile reaction products. Oxide/metal etch selectivities are typically of the order of 10:1, but some of the useful RIE gases also attack photoresist, resulting in poor etch selectivity to resist masks. In addition, etch anisotropy can be poor and the production of unetched residues can result in rough etched surfaces and performance problems. For etching TFMH materials, IBE or RIBE can provide superior etch profiles, and RIBE selectivities and etch rates can be competitive with RIE.
The advantages of “ion milling” or IBE have been described throughout the literature, and include independent control of ion beam energy and current density, and separation of plasma and substrate conditions. That is, the substrate is located outside the plasma which generates the ions, allowing independent optimization of the etching process and the plasma generation process. Energetic, highly directional ions delivered as a broad, high-density beam at the optimum incidence angle(s) to the substrate can produce highly anisotropic etch profiles. Because of the separation of the wafer from the plasma, IBE process pressures are typically two orders of magnitude below those of parallel-plate RIE systems. These low process pressures ensure lossless transport of ion energies to the substrate for maximum anisotropy and limit redeposition of etched material back onto the wafer.
A “pure” IBE process uses inert gases such as argon as the source of etching ions, which may be considered a purely physical etch process. Advantages of this approach are the ability to etch any material, and to etch multi-component materials without residues due to preferential etching. Because no reactive processes are involved, IBE etch repeatability can be well controlled and performance is not sensitive to variables such as wafer preparation. Correspondingly, the lack of a chemical etch component may also limit the etch rates and selectivities obtained by IBE.
Under certain etch conditions, unacceptable levels of sidewall redeposition (also known as “fences”) can occur for IBE. This is especially a concern when etching metals, particularly noble metals and alloys such as permalloy. Similar problems are observed with RIE of these materials because of the need to use a large physical etch component, as mentioned above. In fact, this effect is exacerbated at normal incidence (RIE configuration), whereas in the IBE process an off-normal etch angle can be used (for example, during the “overetch” stage) to remove redeposited material from the sidewall. It has been demonstrated that production devices can be fabricated in high yield with negligible sidewall redeposition by this method. IBE is presently the preferred method used for patterning the permalloy pole tip in the TFMH industry.
In the RIBE and CAIBE (chemically assisted IBE) method, reactive gases are supplied to the ion source instead of, or in combination with, inert gas. This provides a chemical etch component to supplement the physical etch process. RIBE may be viewed as combining the features of RIE and IBE. Compared to RIE, RIBE and CAIBE provide greater control of the chemical versus the physical etch features. Like IBE, they also provide the capability to easily adjust the etch angle to tailor the sidewall profile or adjust the etch selectivity.
As mentioned, until recently, this technique has had limited usefulness in a production environment because of the fact that most Kaufman ion sources use a DC discharge to generate the plasma. H. R. Kaufman, “Broad-beam Ion Sources: Present Status and Future Directions,” J. Vac. Sci. Technol. A, Vol. 4, No. 3, p. 764 (1986). Broadbeam DC ion sources use hot filaments or hollow cathodes to generate the needed electrons. Hot filaments and hollow cathodes have extremely short lifetimes in corrosive or reducing gas environments. Operation of a DC discharge source at greater than 50% reactive gas is not usually possible even for short periods, because of instability and immediate cathode degradation. Consequently, the reported RIBE etch selectivities have also been limited. These problems are not encountered with an RF-ICP ion beam source such as the one described by V. Kanarov et al., “Thirty Five Centimeter Diameter Radio Frequency Ion-beam Source,” Rev. Sci. Instr., Vol. 69, p. 874 (1998). Since there is no filament or cathode in the plasma, the time between maintenance operations is greatly extended compared to a DC discharge ion source, even for inert gas operation.
While many etching techniques have been employed, current methods do not achieve etching of multiple layer lead overlay structures in a manner that is effective and efficient. There is thus a need for a method of effectively and efficiently etching a multi-layer structure that achieves high selectivity for the lead or other high conductivity material, while achieving low sidewall redeposition and accurate critical dimensions.